Flow induced vibrations (FIVs) have a high interest in many fields of technology and engineering. The main reason for this is that they can have a dangerous effect on structures. They can potentially generate an unacceptable level of oscillations in a structure, which may put the integrity and/or functionality of the structure at risk. Historically, engineers and scientists have therefore tried to prevent FIVs and/or minimize their effects. However, recently it has been shown that some FIVs can be used to extract energy from the flow.
Examples of the present invention are based on the use of a flow induced vibration phenomenon that is generally called Transverse Galloping (TG). Other Flow Induced Vibrations, such as e.g. the phenomenon of Vortex-Induced Vibrations (VIV) or Flutter, have in the past also been considered for energy conversion and electricity production.
Vortex Induced Vibrations:
When an elastic bluff body is under the action of a steady fluid flow, for high enough Reynolds numbers (higher than 50, say) the flow separates from the body surface generating an unsteady broad wake. Typically, the flow pattern is characterized by two shear layers on each side of the body that are unstable and roll up to form vortices. These vortices are shed to the wake periodically with a frequency proportional to the undisturbed flow speed. There is a flow velocity at which vortex shedding has a frequency close to the body's natural frequency of oscillations and, for low enough values of the mass and mechanical properties, significant oscillations can be induced in the body. When the body is oscillating, a complex interaction between the oscillating body and the flow field around it develops. This non-linear resonance phenomenon is known as Vortex-Induced Vibration (VIV). The kinetic energy of the oscillations can theoretically be transformed into electricity through the use of an electrical converter. Examples of energy conversion devices partly or complete based on vortex induced vibrations have been described in U.S. Pat. No. 7,208,845, WO 2012/066550 and U.S. Pat. No. 6,424,079.
At least one disadvantage related to energy converters based on vortex induced vibrations is that only in a relatively narrow range of fluid flow speed, resonance occurs. Thus, relatively good energy conversion efficiency can only be reached in a very narrow range of fluid flow speed. Furthermore, the peak amplitude of oscillation (which directly or indirectly drives the electricity production) is inherently limited.
Flutter:
Flutter is a flow induced vibration affecting streamlined flexible bodies. It is a flow induced instability (not a resonance like VIV) and usually involves oscillations in two degrees of freedom, e.g. transverse to the incident flow as well as torsional (pitch). Flutter generally is a high frequency phenomenon of oscillations of relatively small amplitude. E.g. US 2009/0121489 and U.S. Pat. No. 7,986,051 describe energy converters utilizing flutter.
One disadvantage related to energy conversion based on the phenomenon of flutter is that it is a high frequency phenomenon which causes oscillations of small amplitude. This in itself can make electricity production complex. Moreover, the influence of the Reynolds number (which indicates the ratio of inertia forces and viscous forces in the flow) in the oscillations is very significant. This means that relatively good energy conversion efficiency can only be reached for specific flow conditions.
Galloping and Transverse Galloping:
Galloping is a well-known phenomenon in the field of Civil Engineering. It can commonly be observed in high-tension electric transmission lines when the ice accretion on the wires modifies the original substantially circular cross-sections of the transmission lines. A wake caused by a first transmission line may cause a downstream transmission line to start galloping. This phenomenon is commonly called “Wake Galloping”.
In slender structures with a relatively low mass, a low damping coefficient, and having a suitable shape or cross-section (such as e.g. rectangular, triangular or an open semi-circular shape (C-shape), or a D-shape) transverse galloping can occur when the velocity of the incident flow exceeds a certain critical value. At flow speeds above this critical value, the stabilizing effect of structural (mechanical) damping is overcome by the destabilizing effect of the fluid force, and a small transverse displacement of the body creates a fluid force in the direction of the motion that tends to increase the amplitude of vibration. Once the instability threshold is exceeded, an oscillatory motion (which is mainly transverse to the flow) develops with increasing amplitude until the energy dissipated per cycle by mechanical damping balances the energy input per cycle from the flow.
Amplitude and frequency of steady oscillations depend on the geometrical and mechanical properties of the body (cross-sectional shape, mass, natural oscillation frequency and mechanical damping) as well as the incident flow velocity. With increasing flow velocity, the amplitude of the oscillations increases keeps on increasing, and at least in theory, there is no upper limit.
One significant advantage of Galloping for energy conversion and electricity production with respect to e.g. Vortex Induced Vibrations, is that the oscillations do not merely occur in a limited range of flow velocity, but instead they occur at any flow speed above the critical flow speed. Furthermore, the amplitudes of the oscillations in Galloping may be relatively large whereas in Vortex Induced Vibrations they may be relatively small. The ratio of amplitude/characteristic length of the galloping body may reach 3 or 5 or 10. The characteristic length of the body may e.g. be the width of the cross-section of the body. For vortex induced vibrations, a maximum possible amplitude may be given for this ratio around 1.
Another relevant aspect is that the flow speed at which transverse galloping occurs depends strongly of the mechanical properties of the galloping body. As a general rule, the lower the mass and mechanical damping are, the lower the critical flow speed is for galloping. For the purpose of energy efficiency, this is a great advantage, because to some extent one can control (e.g. by of the appropriate choice of mechanical properties) the flow speed at which galloping will take place.
Furthermore, Transverse Galloping is much less dependent on Reynolds numbers than other flow induced vibrations herein described. This means that the energy converters based on Transverse Galloping may more easily be increased in scale.
Previous theoretical studies developed by the inventor Barrero-Gil (Barrero-Gil et al. Transverse Energy harvesting from Galloping, Journal of Sound and Vibration 329.14 (2010), 2873-2883) have highlighted that, in principle, it is possible to efficiently transfer energy from a fluid flow to a prism.
JP 2006-226221 discloses energy converters based on transverse galloping. A vibration body is cantilever mounted and electricity generation is based on deformation of piezo electric materials. At least one disadvantage related to this arrangement is that it would be difficult and/or expensive to reach a large scale energy conversion.
Previously, “Hydroelastic Oscillations of square cylinders”, Denis N. Bouclin, University of Toronto, 1975, studied the interaction between vortex shedding and galloping type oscillations of square cylinders immersed in a water stream. Bouclin did not recognise the possibilities for energy extraction from a fluid flow using transverse galloping. Rather, the idea behind Bouclin's research was to examine the phenomenon in order to be able to avoid negative effects of transverse galloping. Even if the possibility of energy harvesting had been recognised, the experimental set-up would hardly have been useful for this purpose. In the experiments carried out by Bouclin, an oscillating square cylinder is guided along a guide using air bearings with relatively low mechanical damping coefficient. The square form of the cylinder is also not optimized for transverse galloping. Furthermore, the ratio of densities m*=ρgalloping body/ρfluid would not have been suitable for this purpose either.
The present disclosure relates to various methods and systems for improving energy conversion (and electricity production) based on Transverse Galloping vibrations.